Popular Science Monthly/Volume 47/June 1895/Irritability and Movement in Plants
By D. T. MACDOUGAL.
WITH the extension of information concerning the scope, purposes, and adaptations of the movements exhibited by plants, the determination of the nature of the specific forms of irritability under which these movements are induced becomes a question of very great interest. The more apparent movements of plants have long been matters of common observation, yet no apprehension as to their real nature existed before the middle of the present century. Previous to that time natural philosophy was chiefly busied in the definition of the "distinctive qualities" of the great groups—plants, animals, and minerals—and perpetuated without serious inquiry the beautiful vagaries of Aristotle as to the possession of a materialistic soul by plants. Both before and after the dictum of Linnæus, "Lapides crescunt, vegetabilia crescunt et vivunt, animalia crescunt, vivunt et sentient" (1735), the literature of natural history is rich in allusions to the possessions of functions by plants corresponding to the senses of animals. The only actual distinction made by Linnæus between the two groups was the denial of ideal perception to plants. Erasmus Darwin, in his Zoönomia (1794), supposes that plants possess voluntary power. He says: "The sleep of animals consists in the suspension of voluntary motion, and as vegetables are subject to sleep there is reason to conclude that the various actions of opening and closing their petals and foliage may be ascribed to a voluntary power; for without the faculty of volition sleep would not have been necessary for them." Probably a fair representation of prevalent thought on this subject at an early part of the present century is made by the admirable treatise of Tupper, in which he attributes to plants irritability, a form of instinct (to account for spontaneous movements), and a low form of sensation, which he supposes might be accompanied by a degree of consciousness—"and as sensation does exist in animals independently of those eminent attributes with which it is combined in our natures as rational agents, may we not reasonably infer that vegetables have likewise their share of sensitive power, and consequently the means of enjoying their existence?" Hence, as vegetables are necessarily so different from animals in their mode of existence, it is very evident that we can not form any idea how they feel under any circumstances; but we are not on this account to conclude that they are destitute of every kind of sensation. "As they possess life, irritability, and motion, spontaneously directing their organs to what is natural and beneficial to them, and flourishing according to their success in satisfying their wants, may not the exercise of their vital functions be attended with some degree of sensation, however low, and some consequent share of happiness? (Vide Smith's Introduction to Botany.)"
Biological literature even in recent years abounds in expressions concerning phenomena of plant life corresponding to the sensorial action of animals. These conceptions were fostered to some degree by Charles Darwin's Power of Movement in Plants and other works, in which the actions of plants are described in terms strictly applicable to the sensorial reactions of animals only. Thus he says of the irritability of the tips of roots, "It is hardly an exaggeration to say that the tips of the roots affected and having the power of directing movement in the adjoining parts act like the brain of the lower animals." This phraseology was, no doubt, intended to be suggestive rather than definitive, but to it may be traced many current erroneous impressions. "Instinct," "intelligence," "nervous action," and a score of similar terms are used indiscriminately to designate actions of plants far removed in character from those denoted by the original meaning of such expressions. A partial justification of this misapplication of terms is found in the lack of systematically arranged information concerning the form of sensibility exhibited by plants. With an extensive nomenclature dealing with the great mass of detail of the neuro-muscular action of animals at command, the apparent similarity between the irritation reactions of plants and the sensorial reactions of animals has been held to be real, in a manner strongly suggestive of the anecdote of the German peasant who, seeing a moving locomotive for the first time, exclaimed, "There's a horse inside of it, or how could it run?"
The conception of the relative character of these two great groups of reactions may be attained by outlining the conditions which have led to the development of each rather than by an accentuation of external similarities.
Protoplasm, the physical basis of animal and plant life, has among other characteristics that of irritability to several classes of stimuli furnished by its environment. It reacts to these stimuli by adjustments, many of which are accomplished by motion or contraction, and to others by metabolic changes. What agencies have been potent in the development of this primal irritability of protoplasm, through reflex action, into sensorial reaction in the animal, and into the various forms of specific irritability in the plant?
It is unquestionable that the paramount necessity for every organism is that of self-preservation. To obtain food, avoid injury, and secure the proper degree of the environmental conditions of light, temperature, and moisture, are then to be considered as the fundamental necessities of every organism. Animals are organisms in which destructive metabolism prevails, in which more energy derived from complex foods is dissipated than is conserved. Connected with and underlying this condition of the metabolic balance is the fact that animals have steadily developed toward motile forms. In the accomplishment of the conditions of life, motion has become to them an indispensable function. The necessity for the ability to direct the locomotory movements in the avoidance of danger and the attainment of food and comfort has led to the development of irritability into the forms of sensorial action. Plants, on the other hand, are organisms in which constructive metabolism prevails, in which more energy is conserved than is consumed in the performance of the necessary work. Underlying this state of the metabolic balance is the fact that plants have steadily moved along a line toward fixed forms. Consequently irritability has been developed into forms which would be of service to the plant in securing food and protection without moving from place to place. Not only has the protoplasm of the plant developed an irritability to different qualities of the stimuli to which animal protoplasm responds, but it also reacts to certain forces to which the animal is inert, by a mechanism different in every essential from that of the animal. These two lines of development of the primal irritability of protoplasm by reason of the metabolic activity and other conditions attendant on each are so widely divergent that great care must be exercised in the comparison of the higher forms of sensibility exhibited by the plant and the animal. In the animal the higher form is that of sensorial reaction with its vast range of usefulness. In the plant this power has developed with equal facility for the necessities of the organism, but even in its highest form it must needs be termed irritability, although it has attained a degree of specialization in which its delicacy and usefulness are equal to those of sensorial action, and may surpass them in some instances.
Among the more marked forms of irritability shown by the plant may be mentioned those by which it reacts to gravity, radiant energy (heat, light, and electricity), shock, and metabolic action. A clear conception of the character of the reactions of plants to these stimuli may be attained by a consideration of those shown toward gravity, light, and contact of solid bodies.
Since the plant is a fixed organism and can not move in search of food, it is essential that its roots thoroughly penetrate the soil of the locality in which it is found, in such manner as to place its absorbing surfaces (root hairs) in contact with whatever nutrient solutions the substratum may contain. The necessity for such penetration of the soil has led to the development of irritability to gravity in the protoplasm of the roots. Primary roots in response to the stimulus of gravity tend to place themselves in a position with their axes lying parallel to the force of gravity and the tips pointing vertically downward. But if all the roots assumed this position they would depend from the stem in a compact mass in a manner not advantageous to the plant. The secondary or smaller roots, however, react to gravity in such manner that they place their axes at right angles to the line of force, thus securing a penetration of the soil in a second direction. These forms of reaction to gravity are also exhibited by other organs Fig. 1.—Downward Curvature of a Primary Root of Pisum which has been placed in a Horizontal Position. of the plant but do not occur to any extent among animals. The manner in which a primary root curves to place its axis in a vertical position may be seen in Fig. 1, and that of a culm of grass to become erect in Fig. 2.
An important requirement of aërial organs is that they assume a position in which their surfaces will be exposed to the sunlight at an angle most advantageous for the performance of their functions of the formation of food and transpiration. To meet this need, those portions exposed to the light have acquired a specific manner of response to the light by which some place their surfaces parallel and others at right angles to the direction of the rays. Gravity acts continually and invariably in one direction, and with a constant force. Consequently the movements of plants in response to this stimulus are comparatively simple. Light has its source in the sun, which varies its position through one hundred and eighty degrees during the daytime and is wholly
Fig. 2.—Upward Curvature of a Culm of Grass which has been placed in a Horizontal Position. The dotted outline denotes the original position of the plant.
absent at night. In consequence the movements of the plant to adjust its surface to this stimulus of ever-varying direction and intensity are of great complexity. This variability of the stimulus has, moreover, induced in the plant a delicacy of irritability toward light far beyond that exhibited toward gravity. The manner in which leaves react to light may be seen in Fig. 3.
Fig. 3.—Diagram of Light Position of Leaves. The arrows indicate the direction of the rays which have fallen on each plant separately.
In a large and varied category of plants it has become of great importance that they execute certain movements when solid bodies come in contact with them or strike their surfaces. This irritability to contact or impact has been developed in a number of carnivorous plants, which entrap and hold insects which serve as food; in the tendrils of climbing plants, which coil around supports and lift the foliage and flowers into sunlight; in a large class of "sensitive plants," which quickly fold their leaves on the reception of such a stimulus, thus avoiding injury from hail or grazing animals. The need of delicacy is much greater here than
Fig. 4.—Tendril of Passiflora Forty Seconds after Contact of Wooden Rod against Lower Surface. The dotted outline designates the original position of the organ.
in the previous forms described, and the response is much more marked and rapid. As an example of this form of irritability may be cited the contact reaction of tendrils. In Fig. 4 is shown the curvature of a tendril forty seconds after it has been lightly touched with a wooden rod.
With the general features of these reactions at hand, the question as to the mechanism by which they are accomplished becomes one of very great interest. The first point which naturally presents itself is the reception of the stimulus by the plant. Is every cell in the plant or organ affected by the stimulus, and do all bear an equal and similar part in the resulting movement? It is found by experiment that if the terminal region (see Fig. 1) of a root is cut away with a sharp razor, the root will no longer respond to gravity, but will remain in whatever position it may be placed. After a time, when the tip has been rehabilitated, the root regains its power of response to this force as before. The results of this and other experiments tend to show that the only part of the root which can receive the stimulus of gravity is a small mass of cells in the center of the tip. It may be seen, further, on reference to Fig. 1, that the curvature occurs in the fourth and fifth divisions from the tip. Here, then, is an instance in which a distinct mass of cells—"the perceptive zone"—receives the stimulus, and curvature results from the action of another mass of cells—"the motor zone"—several millimetres distant. This would, of course, imply that the impulse from the stimulus received by the perceptive zone was transmitted to the motor zone, and that the reception of the stimulus, the transmission of the impulse, and motion were each accomplished by separate groups of cells. The receiving, transmitting, and motor zones are not always so distinctly separated, however. In stems, leaves, and other aerial organs the perceptive zone may extend over the greater part of the surface, including the region of curvature. In tendrils the perceptive zone comprises the superficial layers of tissue of the concave side of the organ throughout the region of curvature. It is quite probable in all these instances, however, that the cells receiving the stimulus are not identical with those causing the curvature, or, disregarding the cell, separate masses of protoplasm are differentiated for the performance of each of these functions. A clear conception of this mechanism may be attained by a consideration of the structure and action of a tendril. A tendril is generally of bilateral organization, consisting of a middle layer of pith between two layers of elastic and flexible mechanical tissue (wood); outside the wood, on both the upper and lower sides of the organ, layers of mobile thin-walled parenchyma cells, which in the active condition of the tendril are in a state of high internal tension; covering the parenchyma, a layer of epidermal tissue composed of elastic, thick-walled, but easily extensible cells (see Fig. 5). If a solid body is brought into contact with the concave
Fig. 5.—Diagram of the Longitudinal Section of a Tendril in the Region of Curvature. a, epidermis (perceptive zone); b, parenchyma cells which before curvature were similar to f in form; c, e, wood; d, pith; f, parenchyma slightly more elongated than before curvature; g, epidermis of convex side.
side of a tendril, the pressure acts as a stimulus on the ectoplasm—the thin layer of protoplasm lining the walls—of the epidermal cells. The ectoplasm of these cells is connected with that of the thin-walled parenchyma cells at numerous points by means of very fine strands of protoplasm extending through the walls of the intervening cells. An impulse is conducted along the strands to the protoplasm of the thin-walled cells, and their action results in the curvature of the organ. The structure and action of the tendril are generally similar to those of a large number of stems.
The manner of action of the motor cells (Fig. 5, c, e) has not been clearly made out. By different investigators the curvature has been successively attributed to an accelerated growth of the convex (upper) side of the tendril, a retarded growth of the concave (lower) side, an increased turgor in the region of curvature with an increased extensibility of the walls of the convex side, and an increased turgor of the convex side only, coupled with an increased extensibility of the walls of the cells on this side. But recently the hypothesis that the curvature is due to the increased turgor of the concave side, with an increased extensibility of the walls of this side in one direction only, has been offered. Whether the last offers the true solution of the problem or not remains to be proved. However, it accounts for certain features of the curvature irreconcilable with previous explanations. The conditions of the motor cells after curvature may be seen in Fig. 5.
In plants provided with cushions of tissue—pulvini—for the purpose of rapidly displacing leaf stalks and other organs, the movement is effected by a direct contraction of the cells on the side toward which the organ is curved. It will be quite remarkable if it should be found that the rapid movements of the "sensitive plants" are effected by the contraction of the motor cells, and the slower movements of tendrils and other organs by the expansion of these cells.
Of the ultimate molecular changes ensuing in the cells of the motor or perceptive zones, as well as in the transmitting tissues, nothing is known, except that in such cells the metabolic action is necessarily very rapid. In this the physiologist confronts a problem which may not reach its final solution until the ultimate organization of protoplasm is at least approximately ascertained.
The delicacy of the mechanism of irritability in certain instances is such that the amount and intensity of the stimulus necessary to secure a reaction are extremely small. It has been found that tendrils would respond to the contact of a weight not greater than one five-thousandth of a milligramme, and that a plant in a dark chamber would curve toward the light afforded by a single spark from a small condenser coil, or to a light too diffuse to cast a shadow perceptible to the human eye. Some of the lower free-swimming forms have been found susceptible to the presence of an amount of oxygen so small as the one-trillionth part of a gramme, which is very nearly the atomic limit of this substance. This delicacy of perception is doubtless beyond that of the senses of the higher animals.
At all times the amount of a stimulus necessary to produce a response increases with the amount previously acting upon the plant. Thus a plant in total darkness will react to an extremely small amount of light, as has been pointed out; but, if placed in a strong diffuse light coming from all directions, it will be found unresponsive to an increase of the light from one side far in excess of the amount just described. Pfeffer found that, While the spore tubes of ferns were ordinarily affected by an extremely small amount of sodium malate, yet, when placed in a one~hundredth-per-cent solution of this substance, would not respond to an additional amount of the substance until it reached a concentration thirty times as great. The results obtained by other investigators indicate that Weber's law is applicable also to the reactions of plants to light, and perhaps all forms of stimuli.
When a stimulus impinges upon a perceptive zone, the reaction shown by the motor zone does not occur simultaneously with the reception of the stimulus, but after a "latent period" of varying duration.
In Fig. 6 are graphically represented the features of the movement of a tendril which has been stimulated by the contact of a solid body (see Fig. 4). Thirty seconds elapsed after the stimulus was applied before the movement began. The contraction
Fig. 6.—Curve of contraction of Tendril. The distance of the curve from the base represents the amount of displacement of the hip; ﬁve centimetres on the base line represents ﬁve minutes; 1 to 2. latent period and period of contraction; 2 to 3, period of maintenance of contraction; 3 to 4, period of relaxation.
then went forward with, at first, an accelerating, then a decreasing, rapidity until the maximum of contraction was attained. This position was maintained several minutes, when a relaxation occurred by which the organ was restored to its original position. In comparison with the long familiar reaction of the frog muscle-nerve preparation, it will be seen that in one the latent period is about one hundredth of a. second, in the other thirty seconds; the period of contraction in one is ﬁve hundredths of a second, and in the other twelve hundred seconds; the period of maintenance of contraction in one is momentary, and in the other it endures eighteen hundred seconds; the period of relaxation is live hundredths of a second in one, and thirty-three hundred seconds in the other.
The essential features of the mechanism of each of the two movements are so widely different that these are not capable of strict mutual interpretation, but in general the rapidity of contraction in plants may be said to be less than in animals. It must be borne in mind that the mechanism of each has developed the rate of movement best adapted to its needs, and that this rapidity of movement is, furthermore, conditioned by the metabolic activity of the organism which supplies the necessary energy.
In recapitulation it may be said that the primal irritability of protoplasm has been developed in the motile, destructively metabolic animals through the phases of reflex into sensorial action, while in the fixed, constructively metabolic plants the main line of advance has been, not toward sensorial action, nor along a line parallel to it, but at a wide angle from it, into a series of highly specialized forms, whose delicacy of mechanism and efficiency is as commensurate with the complex necessities of the plant as those of the sensorial apparatus to the animal.